Some things do indeed improve, but some other things require a massive effort that no hobbyist or small company can make happen.

This is most obvious in places where a lot of coordination is required, for example in supporting proper color correction throughout all applications, or decent support for advanced printer functions.

There are many incremental changes, but we often get stuck in local minima for years.

Still, I personally like that one can (relatively) easily watch what happens under the hood. It's not entirely clear to me why Windows and MacOS must remain closed source.

Because if it's open source you can't make money from it or have multiple tiers.